This invention relates to circularly polarized antennas and more particularly to circularly polarized antennas for use in FM radio or in television broadcasting where the antennas are mounted on the top of a support tower and about a support mast which may be of conductive material.
Although horizontally polarized television broadcasting has been almost exclusively used in the United States, it appears from some recent test results that circularly polarized broadcasting might well greatly improve television reception both in large metropolitan areas and in fringe areas.
This invention provides an antenna for broadcasting circularly polarized signals and which, when mounted on a support mast, radiates these signals in an omnidirectional pattern about the mast such that when this mast is erected in the center of a city, for example, substantially equal coverage is provided about the city. The problem of equal coverage about the mast becomes increasingly difficult with conventional antenna systems as the diameter of the mast becomes larger with attendant cloverleaf radiation patterns. These tower diameters tend to become fairly large if the tower itself must be fairly high or support many antenna systems for a plurality of broadcasters. The problem becomes increasingly difficult when this omnidirectional pattern is in the circularly polarized mode.
A low cost antenna for radiating substantially circularly polarized signals over a given range of frequencies omnidirectionally about and substantially broadside a support mast has been provided by four conductors helically wound about and spaced from the mast a given radial distance from the center of the mast with each conductor spaced about 90.degree. of arc about the mast from the adjacent conductors. The conductors are fed with equal power signals at a frequency within the given range of frequencies so that in a plane perpendicular to the axis of the mast the phase of the signal at one conductor is 180.degree. out of phase with the phase of the signal at the two adjacent conductors and in phase with the signal at the alternate conductor. The pitch angle of the conductors for the given radial distance is selected to radiate circularly polarized signals substantially broadside the support mast. An antenna of the type just described is the subject of applicant's application Ser. No. 522,132, filed Nov. 8, 1974, now U.S. Pat. No. 3,940,772 and entitled "Circularly Polarized, Broadside Firing, Tetrahelical Antenna." It is desirable in situations such as when the mast is large that the helix radius be larger and that more than four conductors be distributed about a mast to achieve a substantially circularly polarized and substantially broadside firing antenna.
An elliptically polarized helix antenna is described in a patent No. 3,906,509 to Raymond H. DuHammel entitled "Circularly Polarized Helix Spiral Antennas." This patent discusses elliptically polarized helix antennas and while it suggests that helices be wound about the mast and discusses mode numbers, it requires that in determining the number of helices that 2 M/N should not equal an integer where M = the mode number and N = the number of helices. (See column 10, lines 32 thru 50)